The invention relates to a security device for use in securing documents and other articles of value against fraudulent reproduction, counterfeiting and the like.
Many security devices are based on the use of optically variable effect generating structures which generate holograms and the like since these are difficult to manufacture. Examples of such holographic structures and their manufacturing techniques can be found in EP-A-0548142, EP-A-0632767, and WO-A-99/59036 all owned by De La Rue International Limited and the teaching of which and other patents referred to within these documents are incorporated herewith by reference. WO-A-92/09444 is also incorporated as part of the prior art which teaches how to create an improved durability secure and simple to authenticate optical microstructure image feature for visual authentication of a banknote. Certain visual diffraction grating security devices already exist, for example as described in EP-A-0105099 which describes a security device showing an apparent movement effect consisting of areas of plane diffraction grating orientated at different directions along a track each of which diffracts an incident light beam into one particular direction although it should be noted that these are purely diffractive grating devices, with each segment purely a diffraction grating and thus incapable of forming a focused out of plane image under coherent illumination as a covert feature.
In terms of previous machine readable or coherently viewable holographic security structures we would refer to EP-A-0548142 for techniques on how a hologram may create an out of plane image for authentication, although we would point out that in this case the machine readable structure was designed to be completely hidden from visual view to an observer of the hologram and in fact consisted of superposed weak diffraction gratings which did not form a focussed out of plane image. DE-A-3840037 shows an example of a visual security hologram containing a superposed additional laser transmission hologram designed to form an out of plane image to be revealed under laser light with a machine reader or visualiser and designed to be non-recognisable under normal lighting.
There is a continuing need to enhance the security of such security devices while enabling them to be readily authenticated.
U.S. Pat No. 5,825,478 describes a system and method for determining which of a plurality of visually indistinguishable objects have been marked with a covert indicator in which a portion of a surface of each of a first type of objects are provided with a covert holographic indicator which is exposed to be viewed but which is detectable only when illuminated with a coherent reference light of predetermined wavelength.
In accordance with the present invention, a security device comprises a holographic optically variable effect generating structure having at least two discrete sections which generate In response to white light illumination an optically variable image consisting of at least two defined graphical elements located at or near an image plane either on or adjacent to the plane of the device, and which, in response to coherent illumination, generate at least two discrete covert images, in the form of indicia, whose image planes are located at a distance away from the physical plane of the device, the covert images being reconstructed at different angles to a normal to a substrate supporting the device such that the covert images are spatially separated on their image plane, and the covert images being substantially non-visible under white light illumination.
By holographic optical variable effect generating structure we mean, In this context, any diffractive device with the property of forming a first visual localized graphical image near the real plane of the device for visual observation and which also forms an out of plane covert laser verifiable image (instead of the more usual rainbow slit) for viewing under coherent light. Such a structure can only be created by holographic means, or by computer calculation and direct writing of the required fringe pattern of the structure which represents a slower and much more time consuming method of creating such an element.
A particularly useful form of holographic optically variable effect generating structure is one where the replay directions of the visual optically variable image are designed so as to generate an apparent movement effect on tilting the device around a particular axis.
This new holographic security device comprises a structure which generates at least two defined localized optically variable graphical images in the image plane adjacent the structure in response to white light illumination and at least two covert images spaced from the image plane in response to coherent illumination, and located at different angles to a normal to a substrate supporting the device. The covert images will generally be separated by a distance of the order of half their size. Thus, under normal white light illumination, the observer will see a typical optically variable image such as a hologram or diffractive effect consisting of at least two or more parts but under coherent illumination such as laser light, the observer will see two or more different covert images.
These covert images are preferably in the form of graphical elements, logos or alphanumeric characters which may typically be related to the article or document with which the security device is provided. The visual optically variable images are also preferably in the form of small defined shapes or indicia.
In the preferred case, the covert images are visible to the naked eye when reconstructed under coherent laser illumination of the device, although they could also be constructed at a wavelength outside the visible region for machine detection.
In one embodiment, the optically variable effect generating structure generates two or more graphical images or shapes in response to white light illumination which appear to move as the device is tilted. In this context, we distinguish between the apparent three-dimensional appearance of a hologram or the like with a movement effect such as a lateral movement generated by different diffraction angles between elements.
Conveniently, in this embodiment, each element of the holographic optical variable effect generating structure is formed as a single structure with typically a maximum lateral dimension not exceeding 1.5 mm. Preferably, the maximum lateral dimension is not less than 0.5 mm and most preferably in the range 0.5-0.75 mm, there being at least two such elements, replaying different covert images in different directions within the device.
The reconstruction of the covert images can be enhanced by providing a number of pairs or sets (of three or more) of these sections, each member of a pair or set being arranged to generate the same covert image in response to coherent illumination as the other member(s) of the pair or set. In this case, the individual elements must be located sufficiently close to each other that they fall within the diameter of an illuminating, coherent beam in order for the whole covert message to be reconstructed. Typically, the illuminating beam has a diameter of about 2 or 3 mm using a laser pointer or similar device.
In order to increase security both by increasing the complexity of the overall device and in order to conceal the presence of this new holographic optically variable effect generating structure, the optically variable effect generating structure may be located within a set of further optically variable effect generating structures, designed only to produce white light viewable images or effects, wherein the single and further optically variable effect generating structures, on tilting the substrate under white light illumination, cooperate together to generate a moving image effect. Thus, to the unskilled observer, a moving image effect will be observed under white light illumination and only by illuminating the correct set of optically variable effect generating structures with coherent light will he be able to reveal the covert images. Under normal xe2x80x9cwhite lightxe2x80x9d illumination, it will not be readily apparent to him that there is a difference between all the structures and that additional covert images are contained within the device.
In another embodiment, the optically variable effect generating structure sections are formed as discrete spaced areas.
As with the first embodiment, pairs or sets of these areas may be provided to enhance reconstruction of covert images and each area may typically have a maximum lateral dimension not exceeding 1.5 mm. Preferably, the maximum lateral dimension is not less than 0.5 mm and most preferably in the range 0.5-0.75 mm.
This second embodiment is particularly useful when the structure generates an image in response to white light illumination which moves as the device is tilted. This combination of features is particularly difficult to reproduce but easy to authenticate.
The holographic optically variable effect generating structure exhibiting the out of plane covert images exists as a holographic structure containing two image planes, a visual image plane for white light viewing and an out of plane image plane viewable by illuminating the device with coherent light.
The shape of these discrete sections, preferably graphical elements, is required to form a focus very near to the surface plane of the hologram whereas the information encoded within them forms a focus (or images) sufficiently far in front of this surface plane as to require coherent, for example laser, illumination to view and therefore verify it. Each graphical element when illuminated with said light source will replay its particular information element in a specific and unique angular direction such that when the complete laser verifiable feature is viewed in its imaging or focal plane, each information element has its own well defined and separate viewing zone or area.
For example, the covert images may define a machine readable pattern such as a bar code, each graphical element corresponding to a respective part of the bar code.
In some cases, the discrete sections all have the same simple geometrical shape which is not related or borrowed from the graphical composition of the main holographic image, for example circles (dots), squares or rectangles. However, the area of these said graphical elements should not exceed 3 mm2.
In other cases, the graphical elements may be concealed from the observer by being fully integrated into the main holographic image design. This is achieved by first selecting a minor (as regards its area) but otherwise complete graphical element or entity that forms part of the holographic image designxe2x80x94preferably a graphical design element that is repeated as part of a kinetic or lenticular movement sequence and subdividing that graphical element into two or more discrete parts. Into each of these said graphical parts will be holographically encoded one unit of the information that comprises the laser verifiable feature.
Typically, graphical elements in the form of indicia such as alphanumerics and the like are used.
Preferably, the optically variable effect generating structure is formed as a single continuous structure. However, the elements could be defined by separate sub-structures with spaces between them which do not diffract light. In this case, the elements could be formed in separate steps.
Typically, the device can be incorporated into a security hologram or a secure optical microstructure (e.g. hologram, kinegram, pixelgram, dot matrix structure, stereogram etc.). In this case, preferably the part of the structure which additionally responds to coherent illumination has a grating frequency less than the part of the structure which forms solely a white light viewable image or effect.
Alternatively, it could be used as a stand alone security optical microstructure where only a small area was available, such as a banknote thread, where this feature could provide both public recognition animation or apparent movement and a unique additional covert laser viewable security device. A particularly useful application for this new feature is as an additional public recognition, the covert images allowing public recognition, and security device for application onto banknotes, and potentially other documents of value where the optical microstructure is applied to a non-smooth or non-flat paper substrate, as the technique revealed here provides considerably greater resistance for the laser verifiable structure to paper roughness and crumple than previously known techniques because it is localized over a small area. Applications for the securitisation of branded goods plastic financial cards as an anti-counterfeit security device are also anticipated.
Another possible application of this device would be as a laser verifiable trademark type symbol. For example, many security holographic originations could contain the same apparently simple public recognition feature incorporated into one area, either as a set of dots or graphics providing a simple visual apparent movement effect (e.g. a set of 2 lines of 3 dots moving towards each other tilting) which would become a standardized feature but which under laser coherent illumination could replay entirely different laser viewable messages only revealed under coherent illumination. This could be applied as an upgrade to all forms of security holograms and security diffraction grating devices such as dot matrix devices and more specialised techniques such as the Kinegram (OVD Kinegram Corp.) and the Exelgram (Commonwealth Scientific Industrial Research Organisation (CSIRO), Australia) which would be applied to all forms of security documents and security labels, including those used for brand protection applications and for banknote applications such as threads within paper and holographic stripes and patches applied to paper and other items and substrates.
A particularly useful potential application could be for the window thread, commonly used in banknote and other security papers where although holographic threads are available the area of view in each window strongly restricts the degree of security that can be incorporated into the holographic image. This small window area rill available to reveal a visual image thus making it hard to produce both a simple effective public recognition visual security device and also to incorporate additional optical security in terms of machine readable or laser readable images. This invention would allow a thread to contain a simple publicly verifiable switching or moving pattern (potentially, however, reasonably straightforward to imitate with a dot matrix pattern) which could also replay a secure covert laser verifiable feature. Likewise this feature could be incorporated into a repeated dot or graphic pattern on a banknote stripe or patch to increase the security of banknote optical microstructures as the technique revealed here is much more resistant to crumple, paper roughness and other degradation in use than previously disclosed features.
Another useful but slightly different use of this type of improved machine verifiable device would be as an improved security device on a banknote, building on the teaching of WO-A-92/09444. A useful more secure form of public recognition security device for a secure hologram used on a banknote is to use repeated elements as in WO 92/09444, but instead of treating these in pairs replaying a multiply repeated simple switching image, using one or both of these holographic elements to provide an apparent movement effect, (e.g. a run or rotation) which would be harder to counterfeit for a holographic laboratory and would also provide a useful holographic public recognition animated feature. Within this run of repeated elements, which could be apparently non-overlapping or slightly overlapping to provide for example, a simulated 3D depth effect, or simulated enlarging effect on tilting, some or all of these elements could contain laser verifiable features as revealed in this technique, which would have improved durability and viewable under relatively simple coherent illumination conditions using a laser pointer for example.
Another advantage of preferred embodiments of this invention is that they provide potentially an additional third layer of security check for the security device distinct from that available with prior systems. The first layer of security is the animated visual image produced for visual viewing potentially as part of the security hologram design. The second layer of security is the presence of the covert laser verifiable message, simply revealed even by a relatively unskilled observer using a simple coherent light source, such as a laser pointer and a viewing screen. An additional third level of security would then come by analysing the replay angles and possibly fold planes of the laser viewable image. This could be done for a more sophisticated security check using either a laboratory device or a laser based machine reader or viewer device designed to identify the geometry of replay of the laser verifiable features more precisely then would be possible using the simple laser pen method to provide an additional characteristic security check.